This project uses two rigorous and well-powered behavioral experiments to identify the necessary conditions for obtaining robust and reliable effects of transcranial direct current stimulation (tDCS) on human cognition. We recently discovered that time-of-day is a critical factor when stimulating dorsolateral prefrontal cortex (dlPFC), a factor that has been overlooked and may be responsible for some of the mixed results in the prior tDCS literature. The current experiments are aimed at understanding these time-of-day effects and the extent that they differ in younger and older adults, where time-of-day effects on cognition are known to reverse. To the extent that tDCS is most likely to impact cognition during suboptimal times of day, due to circadian rhythms that naturally lead to suboptimal recruitment of prefrontal cortex, we expect tDCS to dlPFC will have its largest benefit on cognition in younger adults in the morning, whereas tDCS should benefit older adults more in the afternoon. By attending to time of day and using relatively large samples, our study also will provide a stronger test of the idea that tDCS is more likely to improve cognition in older adults, as normal age-related declines in the recruitment of prefrontal processes may cause older adults to be more sensitive to prefrontal stimulation. We also will determine the extent that these benefits of tDCS generalize across episodic and working memory tasks for different kinds of information, thereby informing our understanding of the basic mechanisms through which tDCS to dlPFC can improve cognitive performance in younger and older adults. Finally, we will assess the spatial resolution of the standard tDCS technique by comparing two stimulation sites (left dlPFC vs. left parietal cortex) to sham, and we will determine the extent that a pre-post between-subjects tDCS design minimizes the unwanted effects of individual variability, thereby improving experimental sensitivity to tDCS effects while preserving the validity of the tDCS sham condition. Health Relevance: Because tDCS is the safest and most accessible non-invasive brain stimulation technique available, the identification of experimental factors that can optimize the use of the technique for experimentally manipulating brain function should lead to widespread adoption in future scientific and clinical applications. This research also will illuminate the causal role of dlPFC on the processing of different kinds of information in episodic and working memory, thereby informing our theoretical understanding of these fundamental cognitive mechanisms and how they might be impacted by the aging brain. With this knowledge in hand, future work using tDCS to manipulate brain function will be better poised to make scientific advances, and future work combining tDCS with cognitive training to induce more lasting benefits to cognitive performance will be better poised to test the effectiveness of the training techniques.